3. Evidence of genetic mutations affecting the ADA gene as determined by the CLIA certified laboratory and clinical evidence of combined immunodeficiency based on lymphopenia (absolute lymphocyte counts less than 2SD of age-matched control values) and hypogammaglobulinemia (less than 2SD of age-matched control values) or lack of specific antibody response to vaccination. Evidence of life-threatening illness (increased frequency and/or severity of infections resulting in hospitalization and/or the administration of intravenous antibiotics) is necessary for patients to be eligible under this criterion.